Water flowing through a hose is often used as an analogy for current flowing through a wire. There are many similarities, and most people have sprayed water from a hose or drunk water from a fountain. So the analogy is easy for most people to understand. But this particular thread brings up on of the main differences between water and current, one key way in which the analogy does not work. No, actually two differences.
One is that you can pump water out of one barrel and dump it onto the ground. The water spilling out on the ground will not find its way back into the barrel, and it doesn?t have to. As long as the barrel is not yet empty, you can keep the flow going, with no need for the water that was pumped away to find its way back to the suction of the pump again. Water flow does not require a complete circuit.
Another key difference is the manner in which any given molecule of water discovers that the pump has been turned on, the manner in which each molecule of water feels the force that originates from the pump. It starts with the metal face of the pump?s impeller being in intimate, physical contact with some number of molecules of water. The impeller exerts a force on those molecules, and they in turn push the water molecules next to them, and they all start to move. The molecules of water 10 feet downstream from the pump will not feel the force until the ones 9 feet from the pump have felt it. The push propagates from molecule to molecule. Each ?chuck? of water is pushed by the one before it, and pushes the one after it.
By contrast, an electric current needs a complete path. Here is why. When you close the switch to complete an electric circuit, the power source (generator, transformer, battery, UPS, whatever) establishes an electric field throughout the complete path. Without the complete path, there would be no electric field. That electric field will exert a force on every electron throughout the wires, and it will do so all at once. Every single electron will feel the same force at the same moment. The field pushes the electron that is 10 feet from the battery at the same moment that it pushes the electron that is 9 feet from the battery. One electron is not pushed by the one before it, and it does not push the one after it. All are pushed, and at the same time, and all by the same electric field.
To be more complete in my story, I must admit that the protons in the middle of the copper atoms also feel this same force, and at the same time the electrons feel it. But the electric field (and thus the force) that a battery can generate will never be strong enough to get the protons moving. In fact, it will only be strong enough to move the valence electrons, which generally amounts to only one or two electrons in the outer shell of the atom, depending on whether it is an atom of copper or aluminum or some other conductive material of which the load may be constructed. The force generated from the battery will not move the electrons in the inner shells; it?s just not strong enough. They will feel the force, but they have stronger forces that tend to keep them in their places.
Now let?s talk about drift velocity.
In one particular atom, in the outer (valence) shell, resides an electron named ?Red Fred.? You turn on the circuit, and Red Fred will feel a force. It causes him to jump away from his home atom, and into the atom that is one position further downstream (i.e., in the direction of the push, the direction of what will become the current flow). That atom now has one too many electrons spinning around its center, and it will not be happy about that. An electron is going to have to leave. They are all feeling the same force, but at least one will have to leave. Let us presume, for there is no reason not to presume, that Red Fred is not the one to leave. Red Fred stays with its new home atom, and ?Blue Lou? jumps away. When Blue Lou makes it to the next atom downstream, he winds up stuck there, and Green Jean makes her move to the next atom. Some significant time later, Red Fred will finally be kick out (again, by the same force that got this business started in the first place) and will find a new home in another atom further downstream.
The net effect of this process is two-fold. First, any single electron will merely ?drift? down the wire at the speed of cold molasses flowing down a tree trunk. Thus the term, "drift velocity." However, to a person with a stopwatch and a writing pad who is keeping track of the movement of electrons, there will be no way to tell if the electron that just passed by was Red Fred or Blue Lou or Green Jean or any other. They will all look alike. So the observer is going to say that a large number of electrons just blew past his position in a short amount of time. Let us suppose that the observer counted a total of 6,240,000,000,000,000,000 electrons moving past his position in one second of time (he?s a fast counter). That is essentially the definition of ?one amp of current.?
Now, at last, to answer the original question. When an electron leaves an atom, what happens to the atom? Well an electron from further upstream will fill in the vacant position, for a while anyway. But the jumping around from atom to atom will continue until you open the switch.
One final note. In an AC circuit, the electrons are jumping from atom to atom in one direction for the first half cycle, then they jump from atom to atom in the other direction for the other half cycle. That is the meaning of the ?A? word (?alternating?) in the phrase ?AC? (?alternating current?).
